1. Field of the Invention
The present invention relates to a golf club shaft and more particularly to a golf club shaft, made of a fiber reinforced resin, which is lightweight and improved in its strength and stability of the direction of a hit golf ball.
2. Description of the Related Art
The performance demanded for the golf club is to hit golf balls a long distance and make the direction thereof stabilized. As one means for complying with the demand of hitting the golf ball a long distance, research for making the shaft lightweight is performed. That is, by making the shaft lightweight, the golf club can be swung easily and the head speed thereof increases. Thus it is possible to make the flight distance of the golf ball longer.
As one means for complying with the demand of stabilizing the direction of the hit golf ball, research for suppressing easiness of twisting (decrease of torque) of the shaft is made. That is, by suppressing the twisting of the shaft, it is possible to reduce a deviation of the flight direction of the golf ball which occurs owing to a deflection of the face of the golf club at the time of an impact of the golf ball on the face. Thus it is possible to stabilize the direction of the hit golf ball.
To reduce the torque, in the golf club shaft made of the fiber reinforced resin, the amount of prepregs of an angular layer whose reinforcing fibers are oblique to the axial direction of the shaft is increased or reinforcing fibers having a high tensile modulus of elasticity are used. However, to increase the amount of the prepregs of the angular layer leads to an increase of the weight of the shaft, which prevents the intention of making the shaft lightweight. Because a material having a high tensile modulus of elasticity has a low strength, the shaft is liable to be broken.
To solve the problems, many proposals for making the shaft lightweight and reducing the torque are made. For example, the pipe composed of layered fiber reinforced resins and the golf club shaft are disclosed in Japanese Patent Application Laid-Open No. 8-224809 and No. 2001-62015 respectively. In these proposals, separately from the angular layer formed on the whole length of the conventional shaft made of the fiber reinforced resin, the angular layer is partly formed at only the tip side of the shaft.
However, in the pipe disclosed in Japanese Patent Application Laid-Open No. 8-224809, the outer diameter of the tip of the shaft becomes large because the reinforcing bias layers are disposed at the tip side of the shaft in dependence on the relationship between the mandrel. Thus the pipe has a problem that it is difficult to make the shaft lightweight and a problem that the rigidity at the tip side of the shaft becomes too high. Further if the outer diameter of the tip of the shaft is kept at the conventional diameter (φ8.5 mm-9.5 mm), a sufficient thickness cannot be obtained. Thus a sufficient strength cannot be obtained and thus the shaft is liable to be broken. Another problem is that in the case where a material having a tensile modulus of elasticity not less than 50 ton/mm2 is used to compose the reinforcing layer, the shaft is liable to be broken since the strength of the material is low.
In Japanese Patent Application Laid-Open No. 2001-62015, there is disclosed a method of changing the tapering ratio of the mandrel and layering the angular reinforcing layers one upon another, with ends of the angular reinforcing layers coinciding with the boundary between adjacent tapered parts having different tapering ratios. More specifically, the ends of the angular reinforcing layers coincide with the portion at which the tapering ratio of the mandrel changes greatly from the main tapered part having a large tapering ratio to the gently tapered part having a small tapering ratio. Therefore a stress concentrates on the portion at which the tapering ratio of the mandrel changes greatly and the shaft is liable to be broken.